This invention relates to an electro-optical element which includes a ferroelectric substrate, electrodes provided on principal faces of the ferroelectric substrate and a function section including polarization reversal domains formed in a predetermined shape in the ferroelectric substrate so as to pass a beam of light therethrough, and more particularly to an electro-optical element such as, for example, an optical deflector which makes use of an electro-optical effect.
Conventionally, when to construct a system which has a very small optical system, it is necessary to produce small parts and arrange them in a high degree of alignment.
Meanwhile, as a method of dividing an inputted single beam into a plurality of beams, it is common to use a grating. However, it is sometimes difficult to use the grating since output beams thereof exhibit a considerable dispersion or do not go out in fixed advancing directions.
Further, in order to produce a plurality of function sections with a high degree of accuracy on a same substrate in a same process, an optical waveguide structure is utilized frequently. However, since this method is restricted in that a waveguide structure must be utilized, it is disadvantageous in that it is not easy to utilize it.
The inventor of the present invention has found out that characteristics of the invention according to Japanese Patent Application No. Heisei 7-329851 (hereinafter referred to as preceding invention) filed by the assignee of the present invention in Japan in order to align, when to assemble an optical system including a very small optical system, a large number of different function parts with a high degree of accuracy to integrate the parts. In the following, the preceding invention is described in detail.
In particular, the preceding invention provides an electro-optical element constructed such that it includes a ferroelectric substrate, electrodes provided on principal faces of the ferroelectric substrate, and polarization reversal domains formed in a predetermined shape in the ferroelectric substrate and at least one of domain walls of the domains extends perpendicularly or substantially perpendicularly to the principal faces of the ferroelectric substrate so that a light beam may pass through at least two of the domain walls.
In the electro-optical element of the preceding invention, a voltage is applied via the electrodes formed on the principal faces of the ferroelectric substrate including the plurality of polarization reversal domains to produce a difference in refractive index between the substrate and the domains so that a light beam is deflected by a large amount when it passes through at least two of the domain walls. In other words, as a beam of light successively passes through a plurality of domain walls, it can be deflected by a large amount.
Accordingly, the preceding invention can readily and simply realize an electro-optical element (such as, for example,) a light deflection element (or optical modulation element) of a high resolution having a large deflection angle. Besides, since such deflection is caused by application of a voltage, high speed successive scanning can be performed also by random accessing.
Further, since the deflection based on the difference in refraction index provides a large deflection angle as a result of passage of a light beam through a plurality of domain walls, the electro-optical element exhibits a high effect of convergence or divergence of a light beam and is suitably used as a lens having a variable focal length. This lens can vary its focal length or converge or diverge a light beam in response to an electric signal without involving a mechanical movement.
Further, where the electro-optical element is used as a mirror, since deflection based on the difference in refractive index is caused by application of a voltage, change-over between transmission and reflection of a light beam can be performed by an on/off operation of the voltage. Further, a high reflection efficiency is provided by passage of a light beam through a plurality of domains. The mirror can be caused to appear or disappear with an electric signal without involving a mechanical movement.
Further, with the electro-optical element of the preceding invention, by selecting the shape of domain walls so that at least one of the domain walls extends perpendicularly or substantially perpendicularly to the principal faces of the ferroelectric substrate, the propagation direction of a light beam in the element always becomes parallel to the principal faces. Consequently, a light beam can propagate stably in the element.
The electro-optical element of the preceding invention is preferably constructed such that polarization reversal domains through which a light beam to propagate passes have at least two opposing domain walls to which the beam of light enters at different incident angles and a voltage is applied between the electrodes provided on the opposing principal faces of the ferroelectric substrate to deflect the beam of light in response to the voltage.
Further, the electro-optical element is particularly suitable as the mirror described above where the domain walls are layered in a predetermined period so that a light beam may enter at a predetermined incident angle so that the propagation direction of the light beam may be varied by application of a voltage between the electrodes provided on the two opposing principal faces of the ferroelectric substrate.
Meanwhile, the electro-optical element is particularly suitable as the variable focal length lens described above where at least two domain walls which are convex or concave in the propagation direction of a light beam are present in the propagation direction so that the light beam may be converged or diverged by applying a voltage between the electrodes provided on the two opposing principal faces of the ferroelectric substrate.
Processing of a light beam for two x and y directions which are two perpendicular directions in a section perpendicular to the advancing direction of the light beam can be performed by successively passing the light beam through an electro-optical element which performs processing of the light beam regarding the x direction and another electro-optical element which performs processing regarding the y direction.
In this instance, where a plurality of such electro-optical elements are arranged such that an output end of the first electro-optical element for a light beam and an input end of the second electro-optical element for a light beam are arranged continuously such that the principal faces of the two elements extend perpendicular to each other, a light beam can be deflected or converged or diverged two-dimensionally.
Further, where a plurality of electro-optical elements are arranged via means having a function for converting an x direction and a y direction which are two directions in a section of a light beam perpendicular to the propagation direction of the light beam to each other, the first electro-optical element and the second electro-optical element from among the electro-optical elements are preferably arranged such that the principal faces thereof may extend in parallel to each other.
A light beam may be deflected so that the polarization direction may be perpendicular to the principal faces of the ferroelectric substrate and then deflected so that the polarization direction may be perpendicular to the propagation direction of the light beam and also to the initial propagation direction of the light beam to convert the x direction and the y direction in a section perpendicular to the initial propagation direction of the light beam to each other.
Further, where the electrodes are provided over the overall areas of the opposing two principal faces of the ferroelectric substrate so that the propagation direction of a light beam may be varied in accordance with a voltage applied between the electrodes, the propagation direction of the light beam can be kept stably in parallel to the principal faces of the substrate and unnecessary divergence of the light beam can be prevented.
Where the ferroelectric substrate is formed from crystal of LiNb.sub.x Ta.sub.1-x O.sub.3 (where 0.ltoreq.x.ltoreq.1) and the directions of sides of the domain walls extend in parallel to a mirror plane of the crystal, since the flatness of the domain walls can be improved and deflection can be effected well, this is a preferable construction. It is to be noted that LiNb.sub.x Ta.sub.1-x O.sub.3 represents a substance or mixture which consists of LiNbO.sub.3 (lithium niobate) and LiTaO.sub.3 (lithium tantalate) at the ratio of x:1-x.
Upon formation of the polarization reversal domains, for example, electrodes are provided on the two opposing principal faces of the ferroelectric substrate such that the electrodes provided on at least one of the two principal faces have a predetermined shape, and a voltage is applied between the two principal faces to form the individual polarization reversal domains in the predetermined shape (the polarization is reversed in the desired shape).
Or, the polarization reversal domains are formed in a predetermined shape by irradiating an electron beam or charged articles having negative charge upon a face of the ferroelectric substrate on the negative side of spontaneous polarization (the polarization is reversed in the desired shape).
Or else, the polarization reversal domains are formed in a predetermined shape by irradiating charged articles having positive charge upon a face of the ferroelectric substrate on the positive side of spontaneous polarization (the polarization is reversed in the desired shape).
Where the electro-optical element of the preceding invention employs domain formation means by which electrodes of a predetermined shape are formed on the opposite faces of the substrate and a voltage is applied between the electrodes to reverse the polarizations in a predetermined shape of the electrodes, if the shape of the electrode on at least one of the principal faces is a polygon and at least one side of the electrode extends in parallel to the mirror plane of crystal which forms the ferroelectric substrate, then since the direction of a side of a domain wall into which a light beam is inputted extends in parallel to the mirror plane, the flatness of the domain wall is improved.
In this instance, preferably the ferroelectric substrate is formed from crystal of LiNb.sub.x Ta.sub.1-x O.sub.3 (where 0.ltoreq.x.ltoreq.1).
In the following, the preceding invention is described in more detail in connection with embodiments thereof disclosed in the Japanese patent application mentioned hereinabove. Examples of Light Deflection Element
First, an example wherein the preceding invention is applied to a light defection element which makes use of an electro-optical effect is described.
A beam of light which enters from a medium having a certain refractive index into another medium having a different refractive index is deflected by a refraction between the two media. In this instance, if one of the two media has an electro-optical effect, the refraction angle can be varied by a variation of the refractive index of the medium in response to an electric signal. As a result, the course of the light beam can be bent in response to the electric signal.
Referring to FIG. 19, there is shown a light deflection element 40 according to the preceding invention. The light deflection element 40 basically includes a ferroelectric substrate 1, a plurality of polarization reversal domains 2 produced in the ferroelectric substrate 1, a pair of electrodes 3 and 4 in the form of films applied to two principal faces 43 and 44 of the ferroelectric substrate 1, and an electric signal source 7 for applying a voltage between the electrodes 3 and 4.
The direction of crystal of the ferroelectric substrate 1 (the direction of spontaneous polarization indicated by an upward arrow mark in FIG. 19) and the direction of crystal of the domains 2 (the direction of spontaneous polarization indicated by a downward arrow mark in FIG. 19) are different by 180 degrees from each other, that is, are opposite to each other, as seen in FIG. 19 (and also in FIG. 20). Further, the light deflection element 40 is constructed such that at least one (two in the arrangement shown in FIG. 19) of domain walls 2a and 2b of each of the domains 2 extends perpendicularly or substantially perpendicularly to the principal faces 43 and 44 of the ferroelectric substrate 1 so that a light beam 41 may pass through at least two domain walls of the domains 2 (like 2a.fwdarw.2b.fwdarw.2a.fwdarw.. . . ).
As seen in FIG. 19 (or FIG. 20), the light beam 41 enters the light deflection element 40 via an end face 5 of the light reflection element 40, alternately passes through the ferroelectric substrate 1 and the polarization reversal domains 2 and then emerges from the opposite end face 6 of the light deflection element 40. In this instance, the advancing direction of an emergent light beam 42 is varied by an angle .phi. from the advancing direction of the incident light beam 41 by a variation in refractive index corresponding to the signal voltage applied between the electrodes 3 and 4 by the electric signal source 7.
The magnitude of the variation in refractive index of crystal having an electro-optical effect increases in proportion to the intensity of an electric field applied to the crystal, and if the direction of the electric field to be applied is varied by 180 degrees, that is, reversed, then also the sign of the variation in refractive index is reversed.
Since the substrate 1 and the domains 2 have directions of crystal different by 180 degrees from each other, a difference in refractive index is produced between the substrate 1 and the domains 2 in response to the signal voltage applied between the electrodes 3 and 4, and also the light beam changes its refraction angle in accordance with the difference in refractive index.
FIG. 20 schematically illustrates an example of operation of the element according to the preceding invention. In FIG. 20, two upper figures illustrate variations in refractive index of a domain and the substrate by an electric field applied to the element, and a lower figure illustrates an effect of deflection of a light beam which is increased by forming multiple prisms induced by an electric field. Referring to FIG. 20, after the light beam 41 enters from the substrate crystal 1 into a domain 2 having a reversed polarization, as the refractive index of the domain 2 is varied, that is, the refraction angle of the domain 2 for the light beam 41 is varied, by a signal voltage of the signal source 7, the emergent light beam 42 is deflected from its original light path. It is to be noted that the deflection direction in FIG. 20 is reverse to that in FIG. 19 since the polarity of the voltage is reversed. In this instance, since the beam passes through a plurality of polarization reversal domains 2 as seen in the lower figure in FIG. 20, the light beam 41 which is deflected by a considerably large angle is finally obtained as the refracted angle of the light beam 41 is varied each time the beam 41 passes through a polarization reversal domain 2.
With the arrangement described above, a fine structure can be produced with a high degree of accuracy making use of a semiconductor lithography technique as hereinafter described. Consequently, by producing a large number of prisms for deflection in the form of domains in a single element without the necessity for complicated mechanical working such as cutting, polishing or lamination, a large deflection angle whose realization is impossible with a single prism can be realized by a very simple and easy process without the possibility of exfoliation of prisms in the case of a laminated prism arrangement or of exfoliation of electrode films by application of an electric field.
Further, in the arrangement described above, as illustrated in principle in FIG. 21 in which a single domain is shown, since the domain walls extend perpendicularly to the principal faces 43 and 44 and both of the electrodes 3 and 4 are formed over the overall areas, electric lines 10 of force formed between the electrodes 3 and 4 extend substantially perpendicularly to the principal faces 43 and 44. As a result, the advancing direction of the light beam 41 can be kept stably in parallel to the principal faces 43 and 44 of the light deflection element 40, and unnecessary divergence of the light beam 41 can be prevented.
In contrast, another arrangement shown in FIG. 22 wherein an electrode 3A is patterned in a same shape as a polarization reversal domain 2 acts as a light deflection element wherein the refractive index of crystal of the single domain is varied by applying a voltage only to the single domain crystal. In the arrangement, however, since electric lines 10A of force around end faces of the electrode 3A are curved, the refraction index distribution is distorted and also a light beam 42A is likely to be curved unnecessarily in the depthwise direction of the substrate 1.
FIG. 21 shows an example of a preferred light deflector according to the preceding invention in principle. Referring to FIG. 21, since the electrodes 3 and 4 are coated over the overall areas of the element 40, uniform electric lines of force 10 are realized. Further, by selecting the domain such that domain walls thereof extend perpendicularly to the principal planes of the ferroelectric substrate 1, the boundaries between different refractive index regions can be formed so as to extend perpendicularly to the principal planes of the substrate. Thus, also when a light beam passes through regions having different refractive indices, the advancing direction of the light beam can always be kept in parallel to the principal planes of the element, and unnecessary divergence of the light beam can be prevented.
In the arrangement shown in FIG. 21, by selecting LiNb.sub.x Ta.sub.1-x O.sub.3 (where 0.ltoreq.x.ltoreq.1), KTiOPO.sub.4 (KTP) or the like as a material for the ferroelectric substrate 1 as hereinafter described, a domain having domain walls perpendicular to the principal faces of the substrate 1 can be realized.
Further, when to deflect a light beam, it must be deflected by an equal deflection angle over the overall area of a section perpendicular to the advancing direction of the beam, and to this end, the domain walls must be faces having a high degree of accuracy over the overall area of a cross section of the light beam.
Particularly where LiNb.sub.x Ta.sub.1-x O.sub.3 (where 0.ltoreq.x.ltoreq.1) is to be selected as a material for the substrate, by selecting the direction of a side of a domain wall to which a light beam is introduced to a direction parallel to a mirror plane of crystal, the flatness of the domain wall can be improved.
Subsequently, a method of designing a light deflection element according to the preceding invention is described by way of an example of a lithium niobate (LiNbO.sub.3) substrate.
The shape of the polarization reversal domains 2 formed in the ferroelectric substrate 1 is described. As basic physical property constants of lithium niobate (LiNbO.sub.3), the refractive index n.sub.e for an extraordinary ray whose wavelength is 632.8 nm is n.sub.e =2.200, and the electro-optical constant r.sub.33 is r.sub.33 =30.8.times.10.sup.-12 m/V. The wavelength .lambda. of the light beam is .lambda.=632.8 nm, and the polarization direction is the direction (c axis) of spontaneous polarization of lithium niobate (LiNbO.sub.3).
Here, the variation .DELTA.n.sub.o in refractive index when an electric field E (V/m) is applied between the electrodes 3 and 4 is given by EQU .DELTA.n.sub.o =(1/2).multidot.n.sub.o.sup.3 .multidot.r.sub.33 .times.E(1)
If it is assumed that the emergent angle of a light beam from a medium 1 of a refractive index n.sub.1 into another medium 2 of another refractive index n.sub.2 when the light beam enters at an incident angle .theta..sub.1 from the medium 1 into the medium 2 is .theta..sub.2, then the relationship of them is given, from the Snell's law, as EQU sin.theta..sub.1 /sin.theta..sub.2 =n.sub.2 /n.sub.1 ( 2)
From the expression (2), if it is assumed that an electric field is applied to the electrodes 3 and 4 shown in FIG. 19 so that the electrode 3 exhibits a higher potential and thereupon the refractive index of the substrate 1 becomes equal to n.sub.e +.DELTA.n.sub.e while the refractive index of the domain 2 becomes equal to n.sub.e -.DELTA.n.sub.e, then the deflection angle .DELTA..theta..sub.s (refer to FIG. 23) with respect to the incident angle .theta..sub.1 from the substrate 1 to the domain 2 is given by EQU .DELTA..theta..sub.s =sin.sup.-1 {(n.sub.e -.DELTA.n.sub.e)/(n.sub.e +.DELTA.n.sub.e)}.times.sin.theta..sub.1 !-.theta..sub.1 ( 3)
On the other hand, the deflection angle .DELTA..theta..sub.d (refer to FIG. 23) with respect to the incident angle .theta..sub.1 from the domain 2 to the substrate 1 is given by EQU .DELTA..theta..sub.d =sin.sup.-1 {(n.sub.e +.DELTA.n.sub.e)/(n.sub.e -.DELTA.n.sub.e)}.times.sin.theta..sub.1 !-.theta..sub.1 ( 4)
The deflection angles .DELTA..theta..sub.s and .DELTA..theta..sub.d calculated when an electric field E=500,000 V/m is applied to the element based on the expressions (1), (3) and (4) above where lithium niobate (LiNbO.sub.3) is used are illustrated in FIG. 24A.
From the deflection angles .DELTA..theta..sub.s and .DELTA..theta..sub.d illustrated in FIG. 24A, it can be seen that both of them increase as the incident angle .theta..sub.1 to the domain 2 increases, and particularly, the incident angle is preferably larger than 75 degrees. Here, if the incident angle .theta..sub.1 is selected to 85 degrees, then the deflection angle (.vertline..DELTA..theta..sub.s .vertline.+.vertline..DELTA..theta..sub.d .vertline.) is approximately 0.1 degree.
Further, the other side of the domain 2 should preferably be set in parallel to the light beam so that inputting and outputting of light to and from the same may be minimized. Preferably, the shape of the domain 2 is such a triangular shape as seen, for example, in FIGS. 23 and 25.
If approximately 10 such domains 2 are arranged in a row in the advancing direction of the light beam, then the deflection angle of the light beam increases each time it passes a polarization reversal domain 2, and a comparatively large deflection angle of approximately 1 degrees can be realized finally. FIG. 24B illustrates a relationship between the number of domains and the deflection angle.
Subsequently, it is described that the deflection angle can be further increased by selecting the angle of the emergent end face 6 of the element.
The relationship between the incident angle .theta..sub.1 from the inside of the element to the emergent end face 6 and the emergent angle .theta..sub.2 from the end face 6 to the atmospheric air in FIG. 26 is such as illustrated in FIG. 27 from the expression (2) since the refractive index n.sub.1 of lithium niobate (LiNbO.sub.3) is 2.200 and the refractive index n.sub.2 of the air is 1.
In this instance, if the incident angle .theta..sub.1 is set to a comparatively high value within the range smaller than the critical angle (27.0 degrees), then the deflection angle can be amplified by a large amount. For example, where the incident angle .theta..sub.1 is set to 26 degrees, if the deflection angle of a light beam incident to the emergent end face 6 is 1 degree, then the deflection angle of the light beam emerging from the element can be as large as approximately 6 degrees.
By designing the shape of the polarization reversal domains 2 and the angle of the emergent end face 6 in such a manner as described above, the light deflection element 40 can be realized with a large deflection angle.
The light deflection element 40 according to the preceding invention can be applied to such an optical system as shown in FIG. 28. The optical system shown in FIG. 28 can be applied to various applications such as a display unit, a laser cutting unit or a laser printer which makes use of scanning of a laser beam.
Referring to FIG, 28, in the optical system shown, a laser beam 51 from a He--Ne laser 50 is inputted via a mirror 52 to a 1/2.multidot..lambda. plate 53, by which the phase thereof is adjusted. Then, the laser beam 51 is polarized into a light beam 41 of a predetermined polarization component by a polarizer 54 and then introduced via an iris 55 and a lens 56 into a light deflection element 40 (which corresponds to the light reflection element shown in FIG. 26), by which it is deflected in accordance with a signal voltage from a signal generator 57 applied to the light deflection element 40. The deflected light beam 42 is thus scanned on an object 58 of scanning such as a screen. The signal voltage is amplified to a required level by an amplifier 59, and the value of it is measured by a voltmeter 60.
Subsequently, an example of a method of production of the light deflection element 40 described above is described.
In a process of production of the element, the domains 2 are formed first, and then, the electrodes 3 and 4 are formed, whereafter optical polishing of the end faces 5 and 6 and coating of a non-reflective coating are performed. The individual steps are described in detail.
The polarization reversal domains 2 may be formed in various methods. According to the first method, as seen, for example, in FIG. 29 in which electric field application directions to the ferroelectric substrate 1 of lithium niobate (LiNbO.sub.3) are diagrammatically shown, electrodes 13 of a predetermined shape, for example, a triangular shape are formed by coating of an Al film and a lithography technique on the +z face (+c face) of the ferroelectric substrate 1 of lithium niobate (LiNbO.sub.3), and a plane electrode 14 is formed on the -z face (-c face) of the ferroelectric substrate 1. Then, an electric field of 20 kV/mm or more is applied from a power supply 61 at a room temperature so that the electrodes 13 on the +z face may exhibit a higher potential than the plane electrode 14 on the -z face.
Consequently, a plurality of domains 2 of reversed polarization are formed in a substantially same pattern as the electrodes 13 just below the electrodes 13, thereby forming such an light deflection element 40 as shown in FIG. 19. In this instance, a plane electrode 3 is coated after the electrodes 13 are removed. However, the electrodes 13 may be left as they are while the electrode 3 is coated on the electrodes 13.
It is to be noted that a method similar to the domain formation method by application of an external electric field illustrated in FIG. 29 is disclosed in Masahiro Yamada et al., "False Phase Matching Waveguide Type SHG Element", Journal of Papers of Electronic Information and Communications Society of Japan C-I, 1,994, Vol. J77-C-1, No. 5, pp.206-213. However, since this known method relates to an SHG element, if all electrodes including electrodes for polarization reversal are not removed after formation of domains, then light is attenuated by regions of the electrodes. Accordingly, comparing with such an SHG element just described, the light deflection element according to the preceding invention is much different in that it requires electrodes for variation of a refractive index although the domain formation method therefor is similar.
According to the second method for the domains 2, as seen, for example, in FIG. 30 in which a method which employs irradiation of an electron beam upon the substrate 1 of lithium niobate (LiNbO.sub.3) is diagrammatically shown, a plane electrode 15 is formed by coating of an Al film on the +z face (+c face) of the substrate 1 of lithium niobate (LiNbO.sub.3) and an electron beam 62 of 20 kV (acceleration voltage).times.t (t: thickness (mm) of the substrate 1) or more is irradiated upon each portion of the -z face (-c face) at which a polarization reversal domain 2 is to be formed and is scanned in a room temperature while the plane electrode 15 is grounded.
Consequently, a plurality of domains 2 of reversed polarization (the polarization direction, however, is opposite to that illustrated in FIG. 29) are formed in a predetermined pattern in the substrate 1. Thereafter, the electrodes 3 and 4 are formed on the opposite surfaces of the substrate. However, the plane electrode 15 may be left as it is.
It is to be noted that a method similar to the domain formation method by irradiation of an electron beam illustrated in FIG. 30 is disclosed in M. Yamada and K. Kishima, "Fabrication of periodically reversed domain structure for SHG in LiNbO.sub.3 by direct electron beam lithography at room temperature", Electron. lett., 1,991, Vol. 27, No. 10, pp.828-829. Also this known method is directed to an SHG element.
The two domain methods described above are effective for a ferroelectric material such as LiNb.sub.x Ta.sub.1-x O.sub.3 (where 0.ltoreq.x.ltoreq.1) or KTP.
In the substrate 1 in which the domains 2 are formed, presence of an electric field generated by a distortion stress accumulated in the process of formation of the substrate or an electric field formed by charge injected into the substrate sometimes varies the refractive index of the substrate 1 non-uniformly or makes application of a signal electric field difficult. In order to prevent such a situation, it is preferable to anneal lithium niobate (LiNbO.sub.3) at a temperature higher than 150.degree. C. but lower than 700.degree. C. or anneal lithium tantalate (LiTaO.sub.3) at a temperature lower than the Curie point for several tens minutes to several hours, if possible, in an oxygen atmosphere (or otherwise in the air).
Then, while the electrodes 3 and 4 are formed by coating of conductive films of, for example, Al to the opposite principal faces of the substrate 1 by a vapor deposition method or a sputtering method, the electrode 3 and the electrode 4 must be formed so that they may not be short-circuited to each other.
Thereafter, the substrate 1 is cut into a predetermined shape, and then optical polishing is performed for the end faces 5 and 6. Finally, a dielectric substance or the like is coated in multiple layers on the end faces 5 and 6 by a vapor deposition method or the like so that the end faces 5 and 6 may not reflect a light beam to be used at all, thereby completing the element.
In this manner, a light deflection element which allows high speed random accessing, allows deflection over a large angle and has a high resolution can be realized and besides can be produced simply and readily with a high degree of accuracy.
Example of a Variable Focal Length Lens
Subsequently, an example wherein the preceding invention is applied to a lens having a focal length which can be varied in response to an electric signal.
Also this variable focal length lens makes use of an electro-optical effect and utilizes the fact that, when an electric field is applied to a domain having a crystal direction varied by 180 degrees from that of a substrate, a difference in refractive index is produced between the substrate and the domain in response to the electric field.
In particular, as shown in FIG. 31, the variable focal length lens 70 of the example mentioned above basically includes a ferroelectric substrate 1A, a plurality of domains 2A produced in the substrate 1A, a pair of electrode films 73 and 74 coated the two principal faces 43A and 44A of the substrate 1A, and a light convergence/divergence signal source 77 connected between the electrodes 73 and 74.
The direction of crystal of the substrate 1A and the direction of crystal of the domains 2A are different by 180 degrees from each other similarly as in the arrangement shown in FIG. 19. Further, the variable focal length lens 70 has at least two domain walls, which are formed as convex faces 76a or concave faces 76b in the direction of propagation of a light beam 71, in the propagation direction, such that, when a voltage is applied between the electrodes 73 and 74 provided on the opposing two principal faces 43A and 44A of the ferroelectric substrate 1A, the light beam 71 may be converged or diverged.
The light beam 71 is introduced into the variable focal length lens 70 through an end face 75 of the lens 70, alternately passes the substrate 1A and the domains 2A and emerges as a light beam 72 from an end face 76 of the variable focal length lens 70 on the opposite side. The emergent light beam 72 is converged or diverged in response to a signal voltage from the light convergence/divergence signal source 77.
In particular, the crystal directions of the substrate 1A and the domains 2A are different by 180 degrees from each other, a difference in refractive index is produced between the substrate 1A and the domains 2A in a similar manner as described above in response to a signal voltage applied between the electrodes 73 and 74 and also the light beam is converged or diverged in accordance with the difference in refractive index.
Subsequently, a method of designing the variable focal length lens 70 is described by way of an example of a lithium niobate substrate.
The basic physical property constants and the electro-optical effect of the substrate, the law of reflection and so forth are such as described hereinabove in connection with the light deflection element.
For example, the shape of each of the domains 2A produced with the domain structure described above is designed in such a manner as seen in FIG. 32. In particular, if the radius r of curvature of a lens front side 76a in the advancing direction of the light beam 71 and the radius r of curvature of a lens rear side 76b are both determined to be 20 .mu.m, then the focal length of one lens is given by EQU f=n.sub.s .multidot.r/(n.sub.d -n.sub.s) (5)
where n.sub.s and n.sub.d are the refractive indices of the substrate and the domains when 50 kV/m is applied to the element, respectively.
By substituting EQU n.sub.s =2.2-8.199.times.10.sup.-5 EQU n.sub.d =2.2+8.199.times.10.sup.-5
into the expression (5) above (wavelength .lambda.=0.633 .mu.m), f .apprxeq.27 cm is obtained.
A plurality of such lenses are arranged in such a manner as seen in FIG. 31 so that a light beam may successively pass through the plurality of lenses. For example, where N=300 lenses are involved, the focal length f.sub.n of the entire system is given by EQU f.sub.n =f/N (6) EQU .apprxeq.0.9 mm
This lens 70 has a focal length which varies in response to the intensity of the electric field. For example, if the intensity of the electric field is reduced to zero, the lens 70 becomes fully transparent (does not act as a lens) to a light beam and does not converge nor diverse the light beam.
If the electric field is applied in the reverse direction, then since the refractive index of the lenses decreases on the contrary, the light beam is diverged in response to the intensity of the electric field.
In this manner, with the lens 70 of the present example, the focal length and the situation of convergence or divergence of light can be varied by the direction and the intensity of the electric field. In other words, a lens whose focal length can be varied in response to a signal electric field can be realized.
It is to be noted that the variable focal length lens 70 can be produced simply and readily with a high degree of accuracy by a method similar to the production method for the light deflection element described above. Deflection or Convergence/Divergence of Light in Two-Dimensional Directions
While the light deflection element 40 or the lens 70 described above is directed to deflection or convergence/divergence of light in one-dimensional direction, deflection or convergence/divergence in two-dimensional directions is described below.
As a first method, as schematically shown in FIG. 33, two elements 40 shown in FIG. 19, that is, elements 40A and 40B, are used and disposed such that the principal faces thereof may extend perpendicularly to each other and an output end 6A of the element 40A and an input end 5B of the element 40B may be adjacent each other. It is to be noted that, in FIG. 33, domains described above are schematically shown as 2A and 2B in the elements 40A and 40, respectively, whereas electrodes are omitted.
A light beam 41 is inputted to the light deflection element 40 through an end face 5A of the element 40A. The light beam 41 is deflected or converged or diverged in a y direction in accordance with a signal voltage from a signal source 7A, and then inputted into the element 40B through an end face 5B of the element 40B. Then, the light beam 41 is deflected or converged or diverged in an x direction by a signal voltage from another signal source 7B, and then outputted to the outside through an output end face 6B.
As a light beam successively passes through the two perpendicular elements 40A and 40B in this manner, it can be processed in the two-dimensional directions of x and y.
As another method for processing a light beam in two-dimensional directions, as seen in FIG. 34, an element 80A for x-direction processing and another element 80B for y-direction processing of a light beam (in FIG. 34, a plurality of domains and electrodes arranged in the advancing direction of light are omitted for simplified illustration) are connected to each other such that the principal faces thereof may extend in parallel to each other.
A light beam 41 is inputted to the element 80A for x-direction processing through an end face 85A of the element 80A, totally reflected by another end face 86A optically polished obliquely to a direction perpendicular to the principal faces of the element 80A, and inputted from another end face 85B of the element 80B into the element 80B for y-direction processing. Then, the light beam 41 is totally reflected by another end face 87 of the element 80B optically polished obliquely to a direction parallel to the principal faces 83A and 84A of the element 80A and perpendicular to the advancing direction of the light beam 41 in the element 80A and then propagates in the element 80B in parallel to the principal faces 83B and 84B of the element 80B, whereafter it is outputted as a light beam 42 processed for both of the x and y directions from an end face 86B of the element 80B.
By "bending" a light beam in this manner, the x direction and the y direction of the light beam can be converted. More particularly, in FIG. 34, reference symbols A, B, C and D are applied to the four corners of a section of the light beam on the end face 85A, and rays of light at the four corners are traced. Thus, it can be seen that the x direction (A-D) of the element 80A is converted into the y direction (A'-D') of the element 80B, and the y direction (A-B) of the element 80A is converted into the x direction (A'-B') of the element 80B. It can be recognized that processing for the x direction can be performed by the element 80A whereas processing for the y direction can be performed by the element 80B. It is to be noted that a light beam is deflected in the elements 80A and 80B each time it passes through a domain.
Here, where an element which utilizes polarization reversal of lithium niobate is used, it is convenient that the polarization direction of light is perpendicular to the principal faces of a substrate. Therefore, preferably a 1/2 wavelength plate 88 is interposed between the elements 80A and 80B so that, also with the element 80B, the polarization direction may be perpendicular to the principal faces of the substrate. Accordingly, a light beam can be deflected such that it may be perpendicular to the propagation direction of the light beam and also to the original propagation direction of the light beam to convert the x direction and the y direction of a section perpendicular to the propagation direction of the original light beam to each other.
Example of a Mirror Induced by an Electric Field
FIGS. 35 and 36 show a mirror 100 induced by an electric field, to which the preceding invention is applied.
A plurality of polarization reversal domains 2 are formed, for example, on a lithium niobate substrate 1 by the method described hereinabove. For the domain structure in this instance, a periodical polarization reversal structure wherein the domains 2 are periodically layered from an end face 5 to the other end face 6 of the lithium niobate substrate 1 (in FIG. 36, a reduced number of domains are shown to facilitate understanding). The period is represented by .LAMBDA..
If a predetermined electric field is applied between a pair of electrodes 3 and 4, then where the period .LAMBDA. of the periodical polarization reversal structure is selected so that it satisfies EQU K=2k.multidot.cos.theta. (7)
where K=2.pi./.LAMBDA., k=2n.sub.o .pi./.lambda., n.sub.o =2.200, .lambda. is the wavelength of an incident light beam 41 and .lambda.=0.633 nm, and .theta. is the incident angle of the light beam 41 to a periodical domain plane 90, the light beam 41 inputted at the incident angle .theta. to the end face 5 is Bragg reflected at an emergent angle -.theta. from the periodical domain structure so that reflected light 42A is obtained.
Consequently, if the direction of the domains is selected, for example, so that the periodical domain plane 90 may be in parallel to the incident end face 5, then a light beam inputted at .theta..sub.1 to the end face 5 is reflected at -.theta..sub.1 from the end face 6 only when a predetermined electric field is applied between the electrodes 3 and 4. However, when no electric is applied between the electrodes 3 and 4, the light beam passes as it is through the mirror 100.
Where the incident angle .theta. to the periodical domain plane 90 is, for example, 45 degrees, the period .LAMBDA. of the periodical polarization reversal structure or periodical domain structure should be set to 0.2 .mu.m.
It is to be noted that the electric field induced mirror 100 can be produced by a method similar to that used for the light deflection element described above.
The mirror 100 which can be turned on or off by an electric field can be provided in this manner and besides produced simply and readily with a high degree of accuracy.
While the examples of the preceding invention are described above, they can be modified within the spirit and scope of the preceding invention.
For example, while the shape, quantity, arrangement and formation method of domains described above may be modified in various manners, the object of the preceding invention can be achieved only if domains are formed in a substrate such that at least one domain wall of the domains may extend perpendicularly or substantially perpendicularly to principal faces (on which electrodes are provided) of the substrate and a light beam passes through at least two domain walls (in other words, through at least two locations of a domain wall or walls). It is also possible to form a single domain on which pluralities of faces 2a and 2b are provided in a similar manner as described above as seen in FIG. 37.
In this instance, the element is preferably constructed such that domains through which a light beam passes have at least two opposing domain walls to which the beam of light enters at different incident angles.
As a method of forming the domains, the shape of the electrodes 13 described above may be some other polygonal shape than a triangular shape such as a quadrangular shape or a pentagonal shape, and also the other electrode 14 may have a similar polygonal shape. Further, in place of irradiation of an electron beam described above, charged particles having negative charge may be irradiated, or else, charged articles having positive charge such as, for example, protons may be irradiated upon a face on the positive side of spontaneous polarization, to form domains.
If each of the electrodes for use for formation of domains is formed such that at least one side thereof extends in parallel to the mirror plane of crystal which forms the ferroelectric substrate, then polarization reversal can be performed well and the direction of a side of a domain wall to which a light beam is inputted becomes parallel to the mirror plane, and consequently, the flatness of the domain wall is improved.
It is to be noted that the electro-optical element of the preceding invention can be applied, in addition to light deflectors, lenses and mirrors described above, to various other devices which have domains and exhibit an electro-optical effect such as optical modulators and signal processing apparatus.
Since the electro-optical element of the preceding invention is constructed such that at least one of domain walls of at least two polarization reversal domains formed in a predetermined shape in a ferroelectric substrate extends perpendicularly or substantially perpendicularly to the principal faces of the ferroelectric substrate so that a light beam may pass through at least two of the domain walls, when a voltage is applied via electrodes formed on the principal faces of the ferroelectric substrate, a difference in refraction index is produced between the substrate and the domains. Consequently, when a light beam passes through at least two of the domain walls, it is deflected by a large amount by the electro-optical element, and as the light beam successively passes through the plurality of domain walls, it can be deflected by a large amount.
Accordingly, as an electro-optical element, for example, a light deflector (or optical modulation element) of a high resolution which presents a large deflection angle can be realized simply and readily. Besides, since such deflection is caused by application of a voltage, high speed successive scanning can be performed also by random accessing.
Further, since the deflection based on the difference in refractive index described above provides a large deflection angle because of passage of the light beam through the plurality of domain walls, the effect of convergence or divergence of the light beam is increased. Consequently, the electro-optical element is suitable as a lens having a variable focal length. The lens can vary the focal length thereof or converge or diverge a light beam in response to an electric signal without involving a mechanical movement.
Also where the electro-optical element is used as a mirror, since the deflection based on the difference in refractive index described above is caused by application of a voltage, transmission or reflection of a light beam is allowed by turning on or off of the voltage. Further, passage of the light beam through the plurality of domains increases the reflection efficiency. This mirror can be induced or eliminated in response to an electric signalwithout involving a mechanical movement.
Further, with the electro-optical element of the preceding invention, by selecting the shape of the domain walls such that at least one of the domain walls expends perpendicularly or substantially perpendicularly to the principal faces of the ferroelectric substrate, the propagation direction of a light beam in the element always becomes parallel to the principal faces. Consequently, the light beam can be propagated stably in the element.